(CNN) -- U.S. transportation officials shut down 52 bus companies and 340 vehicles Thursday as part of as an eight-month effort targeting unsafe motor coach operations, according to the Federal Motor Carrier Safety Administration.
Called Operation Quick Strike, the sweeping action from New York to California involved companies in 22 states and the District of Columbia. The companies were put out of business after more than 50 specially trained investigators conducted detailed reviews of safety practices at the 250 most at-risk motor coach companies based on roadside inspection and safety data, according to a statement from the agency.
"Bus travel is increasingly popular because it is a convenient, inexpensive option for students, groups and families," U.S. Transportation Secretary Anthony Foxx said. "But it must also be safe."
Thursday's announcement comes weeks after the National Transportation Safety Board expressed concerns with the thoroughness of FMCSA investigations, citing four deadly crashes involving operators who were already on the radar of the oversight agency.
"While FMCSA deserves recognition for putting bad operators out of business, they need to crack down before crashes occur, not just after high-visibility events," NTSB Chairwoman Deborah A.P. Hersman said in a statement last month.
Two deadly bus accidents in the last year prompted the investigations.
FMCSA shut down the U.S. operations of Mi Joo Tour & Travel of Coquitlam, British Columbia, after an accident in Oregon in December 2012 that killed nine and injured 39. The driver had far exceeded the 70-hour work limit, the investigation found.
The agency also shuttered bus company Scapadas Magicas after an accident in which eight people were killed -- seven bus passengers and a driver in another vehicle -- and dozens were injured. The February wreck occurred east of San Bernardino, California. The company failed to maintain its buses and ensure that its drivers were licensed, the agency said.
"In the aftermath of two deadly crashes earlier this year, FMCSA re-examined the way we investigate passenger carriers to make our methods even more effective at preventing crashes," agency spokeswoman Marissa Padilla said. "Using safety and roadside inspection data, FMCSA identified 250 high-risk carriers for top-to-bottom investigations designed to uncover any patterns of unsafe behavior or faulty bus maintenance."
FMCSA administrator Anne Ferro said Thursday, "I think the most important thing to point out is that we didn't wait for NTSB to tell us we had to re-examine the way we were carrying out investigations. That's exactly what we did early this year, which then led to this thorough sweep of 250 of the highest risk companies."
Dan Ronan, spokesman for the American Bus Association, a Washington-based trade group representing the motor coach, tourism and travel industry, praised the federal initiative.
"The Federal Motor Carrier Safety Administration has done the right thing by finding companies running illegally or in an unsafe manner," he said. "More than half the fatalities and injuries in the industry were caused by a small group of providers. There's no shortage of bus companies. There are plenty of good ones out there."
As a result of the investigation and inspections, 52 motor coach companies were put out of business and shut down for safety violations such as failure to maintain their buses adequately, inadequate drug and alcohol driver testing and overwork of drivers.
Other companies took action to correct the safety violations, and 28 avoided being shut down, the agency said.
In all, 340 vehicles of the more than 1,300 inspected were taken off the road for safety and maintenance violations, according to the report.
Additionally, inspectors looked into more than 1,300 carriers that had little to no inspection history with the safety agency, and more than 240 were will be investigated further.
"This year we evaluated and enhanced our investigation methods to dig deeper than ever before and uncover dangerous patterns of unsafe behavior and business practices," said Anne S. Ferro, administrator of the Federal Motor Carrier Safety Administration.
CNN's Chris Welch and Alexandra Field contributed to this report.